A temperature sensor using the temperature dependency of a forward voltage Vf of a diode is known. FIGS. 1A and 1B are circuit diagrams illustrating the basic configuration of a temperature sensor. As illustrated in FIG. 1A, a temperature sensor 10 includes a diode 12 and a current source 14 for supplying a constant current Ic to the diode 12 and outputs a forward voltage (voltage drop) Vf of the diode 12. The output voltage Vf is expressed by the following equation (1). The diode 12 may be configured using a base-emitter junction of a bipolar transistor, in which case the output voltage Vf may be read as Vbe which is equivalent to Vf.Vf=VT×ln(Ic/IS)  (1)
Where, VT=kT/q, T is a temperature, k is a Boltzmann constant, q is an electronic elementary quantity and IS is a saturated current. It is noted that IS also has a temperature dependency. The temperature sensor 10 illustrated in FIG. 1A has a negative temperature characteristic CTAT (Complementary To Absolute Temperature). The temperature sensor 10 illustrated in FIG. 1A has a problem in that it is affected by variations of Ic and IS.
A temperature sensor 20 illustrated in FIG. 1B includes diodes 22 and 24 and current sources 26 and 28 for supplying constant currents Id1 and Ic2 to the diodes 22 and 24. The current density flowing through the diode 24 is 1/n times as large as the current density flowing through the diode 22. For example, with Ic1=Ic2, the size of the diode 24 is n times the size of the diode 22. Alternatively, with Ic1=n×Ic2, the diode 22 and the diode 24 may be same in size.
The temperature sensor 20 outputs a difference ΔVf (=Vf1−Vf2) between a forward voltage Vf1 of the diode 22 and a forward voltage Vf2 of the diode 24. The difference ΔVf has the positive temperature characteristic PTAT (Proportional To Absolute Temperature) expressed by the following equation (2).ΔVf=Vf1−Vf2=VT×ln(n)  (2)
That is, in comparison with the temperature sensor 10 illustrated in FIG. 1A, the temperature sensor 20 illustrated in FIG. 1B is configured to be less affected by variations of Ic and IS than the temperature sensor 10 illustrated in FIG. 1A.
A temperature sensor is also known, which is obtained by combining the temperature sensor of FIG. 1A or 1B and a ΔΣA/D converter. FIG. 2 is a circuit diagram of a temperature sensor 30 including the temperature sensor of FIG. 1B and a ΔΣA/D converter. The temperature sensor 30 includes a temperature sensor 32, a reference voltage source 34 and a ΔΣA/D converter 36. The temperature sensor 32 may have the configuration of FIG. 1A or 1B. In this example, the temperature sensor 32 is the temperature sensor 20 of FIG. 1B. An output voltage TOUT of the temperature sensor 20 is expressed by the following equation (3).TOUT=β×ΔVf=β×VT×ln(n)  (3)
The reference voltage source 34 generates a reference voltage VREF which is independent of temperature. The reference voltage source 34 is also called a band gap reference (BGR) circuit. It should be understood by those skilled in the art that the BGR circuit is configured with a combination of the temperature sensor 10 of FIG. 1A and the temperature sensor 20 of FIG. 1B. Specifically, when a voltage Vf having the CTAT characteristic and a voltage ΔVf having the PTAT characteristic are added at a rate where their temperature dependencies are cancelled by each other, the reference voltage VREF independent of temperature is generated.VREF=α×Vf+β×ΔVf  (4)
The ΔΣA/D converter 36 converts the output voltage TOUT of the temperature sensor 32 into a bit stream bs. The configuration of the ΔΣA/D converter 36 is well known in the art and includes a subtracter 40, an integrator 42, a quantizer 44 and a one-bit D/A converter 46.
A digital output DOUT of the ΔΣA/D converter 36 is a duty ratio of the bit stream bs and is represented by TOUT/VREF.DOUT=TOUT/VREF=β×ΔVf/VREF  (5)
In the temperature sensor 30 of FIG. 2, the temperature sensor 32 and the reference voltage source 34 are configured separately from each other. Therefore, both of an error and variation of TOUT and an error and variation of VREF are included in the digital output DOUT, which may be a factor of poor accuracy.